1. Field of the Invention
The present invention relates to a process for reducing the production of flyash in a dry bottom boiler. More specifically, the process relates to fusing or combining together many small particles by heating them sufficiently to soften or melt their surfaces and impinging them on each other, or even to melting the small particles together and having the resulting larger agglomerates fall out the bottom of the boiler.
2. Description or the Prior Art
In dry bottom furnaces there has been limited effort to increase the amount of ash being discharged as bottom ash. Neither the flyash nor the bottom ash have commercial uses nearly as large as the supply. The flyash finds some use as pozzolanic material in cement or concrete. Various uses have been found for the bottom ash but they are limited and not as well established as the aggregate or blast cleaning markets of the bottom ash from wet bottom boilers. Usually both the flyash and the bottom ash from a dry bottom boiler must be disposed of in ponds or landfills. Even though the flyash can easily be blown about and causes fugitive emissions at every step of the disposal, few efforts have been made to increase the proportion of the ash which is disposed of as bottom ash from dry bottom boilers.
In dry bottom boilers, the flyash which does not impinge on and stick to water walls, steam tubes or other parts of the boiler and then subsequently fall into hoppers, either passively or as a result of some action of the operator, exits the boiler as flyash. In dry bottom boilers about 80% of the ash is usually thought to leave the boiler as flyash and only 20% as bottom ash. This contrasts to wet bottom boilers, where 80% of the ash is usually thought to exit the boiler as bottom ash and only 20% as flyash, and the bottom ash is expected to flow from the boiler as a liquid. The bottom ash is usually agglomerates of ultimate ash particles which are loosely fused together, but part of the agglomeration may be completely melted together. So complete melting techniques such as described in our U.S. Pat. No. 5,044,286 which are useful in wet bottom boilers would not work in dry bottom boilers. Part of the bottom ash, or slag as the worst of the accumulations are called before they reach the bottom hopper, may be molten and may run or drip, but the normal and desired behavior of wall ash in dry bottom boilers is as solid material. As a solid material, the ash may fall of its own weight or by being blown with strong blasts of air or steam flowing from soot blowers. When the boiler load is reduced in response to low demand at some part of the day or week, or in response to ash accumulations, the ash may fall off because it cools and fractures or the tubes contract and the ash is shed by the differential expansion or contraction. For easy removal from boiler surfaces, it can be seen that solid ash is preferred.
The flyash which exits the boiler in the flue gas stream is usually very small particles. The mass mean average diameter is below 50 micrometers and often below 20 micrometers. The particles are the ultimate ash particles. In order to reduce air pollution by these particles, they are collected in various air cleaning devices such as electrostatic precipitators and baghouses. When the particles are collected in the devices they are often agglomerated, but the agglomerates are neither as big nor as strong as the agglomerates which make up the bottom ash from dry bottom boilers. The bottom ash particles or agglomerates are typically from 100 micrometers to several centimeters in diameter. While the bottom ash is less difficult to handle, there has been little effort to increase the fraction of the bottom ash from dry bottom boilers.
The bottom ash is more desirable from a handling, storage, transportation, and disposal viewpoint since it is not so easily blown about by the wind. Some of it can be used as aggregate material. Most importantly, due to its larger size it will be less of a leaching hazard. The United States Environmental Protection Agency has established extraction tests to determine if coal ash is hazardous. The present procedures are set forth in 40 CFR 260.20 and 260.21. It is emphasized that the test of ash for being hazardous is based on how much of a given element is extractable from a sample, not on how much is in a sample. The sample is crushed to pass a 3/8-inch (9.5 mm) sieve and extracted with water to which acetic acid is added to keep the pH at 5.0. The sample is contacted with the weak acid for 24 hours, after which time the liquid is tested for metals. The extract is tested for arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. A concentration limit is specified for each metal and if one exceeds the specified limit, the ash is considered as having EP Toxicity and considered a hazardous waste. It is well known that disposal of hazardous waste is very expensive and should be avoided if possible.
It is true and recognized by people familiar with the arts of extraction and lixiviation that soluble materials are much more readily extracted from small particles than from large particles. Because small particles have higher surface area/volume ratios than large particles, a higher proportion of the soluble materials are at the surface of the particle and come into contact with the extraction liquid. Therefore, that ash with large particles will often be judged non-toxic while the same ash having small particle sizes would be found to be toxic. Because of their small size the sample crushing procedure specified in the test is not relevant to flyash particles. It would take over 100 million spheres of flyash, which on average is 20 micrometers in diameter, to make one 9.5 millimeter diameter sphere. By increasing the size of ash particles they are made more safe for disposal.
There are four benefits to converting part of the flyash into the larger size bottom ash. They are 1) the bottom ash is not so dusty and does not blow around so much, reducing fugitive emissions and making it easier to handle and dispose of; 2) not having toxic flyash blowing about so much will make working conditions safer; 3) some of the bottom ash may be sold for aggregate or for other uses; 4) the ash will be much less likely to be a hazardous waste, reducing disposal costs.
Procedures have been developed to recycle flyash to boilers in order to burn out the carbon in the flyash and increase the efficiency of the boilers. However, this technique frequently results in the flyash being recirculated a great number of times without any significant increase in the fraction of the incoming ash ultimately leaving the boiler as bottom ash. The ash is simply blown back into the furnace and very little, if any, of it melts. The carbon burns out and the ash leaves the furnace a second, third, fourth or more times as flyash. Such techniques are sometimes applied to dry bottom pulverized coal burning furnaces but are most often applied to stoker furnaces, all of which are dry bottom.